(1) Field of the Invention
The present invention relates to radio communication system and method, and more particularly, to radio communication system and method for performing radio communication control of a system adopting TDMA-TDD.
(2) Description of the Related Art
In radio communications, a large number of users share a common frequency band to communicate with their respective parties, and accordingly, in order to prevent interference of communications between unintended parties, TDMA (Time Division Multiple Access) is widely used on a practical basis in which time is divided for individual users so that an identical frequency may be shared by multiple users.
In PHS (Personal Handyphone System) or SU (Subscriber Unit) which is a fixed subscriber terminal unit installed at home for the purpose of communications, TDD (Time Division Duplexing) transmission is carried out in which transmission and reception are performed alternately in time, using an identical carrier wave. Such TDMA-TDD systems permit efficient use of an allotted frequency band.
FIG. 15 illustrates an arrangement of slots in a TDMA-TDD frame. With respect to the frequency from one radio base station, four channels of transmitting slots TX1 to TX4 and receiving slots RX1 to RX4 are multiplexed in one frame (384 Kbps) of 5 ms long.
To prevent collision of radio signals from being caused due to propagation delay difference, clock jitter or the like, a frame is provided with guard bits g of 16 bits long. Each of TX and RX consists of 240 bits, inclusive of the guard bits g.
SU1, which is located at a short distance from the radio base station, receives down data from the radio base station in a slot time of the transmitting slot TX1, and upon lapse of a fixed time (2.5 msec) from the reception, outputs up data. The radio base station can receive the up data from the SU1 located at a short distance therefrom in the receiving slot RX1.
On the other hand, SU2, which is located relatively distantly from the radio base station, receives down data from the radio base station in a slot time of the transmitting slot TX2 after a delay of a propagation delay time td0.
Accordingly, for the radio base station, the output of up data from the SU2 is delayed for a time (2.5 msec+td0), but since the delay is within the range of protection of the guard bits g as shown in the figure, the radio base station can receive the up data from the SU2 in the receiving slot RX2.
The conventional TDMA-TDD technique described above is, however, associated with a problem that the coverage (service area) cannot be expanded even if the output power of the radio base station or SU is increased.
FIG. 16 illustrates the problem with the conventional TDMA-TDD. SU3, which is located at a long distance from the radio base station, receives down data from the radio base station in a slot time of the transmitting slot TX3 after a delay of a propagation delay time td1.
Accordingly, for the radio base station, the output of up data from the SU3 is delayed for a time (2.5 msec+td1); in this case, the delay is outside the range of protection of the guard bits g, as shown in the figure, and the up data spans the receiving slots RX3 and RX4. Consequently, the radio base station is incapable of reception in the receiving slots RX3 and RX4, with the result that the number of usable time slots decreases.
Thus, the SU3 located at a long distance from the radio base station is unable to communicate with the radio base station, so that data transmission/reception needs to be performed with respect to some other radio base station located at a shorter distance from the SU3.
Accordingly, where a plurality of Sus are installed, the number of radio base stations installed must be increased correspondingly to cover these SUs, but it is desirable from an economical viewpoint that more SUs be covered by the least possible number of radio base stations.